Cylindrical rolls may be utilized in a number of industrial applications, especially those relating to papermaking. Such rolls may typically be employed in demanding environments in which they can be exposed to high dynamic loads and temperatures. As an example, in a typical paper mill, rolls can be used not only for transporting a fibrous web sheet between processing stations, but also, in the case of press section and calender rolls, for processing the web sheet itself into paper.
In the press section of the papermaking process, two or more cooperating rolls press the fibrous web as it travels on felts between the rolls. The “nip” is the contact region between two adjacent rolls through which the paper web passes. Each roll is typically constructed from a metallic shell covered by a polymeric cover. The characteristics of the rolls may be particularly important in papermaking, as the amount of pressure applied to the web during the nip press stage can be critical in achieving uniform sheet characteristics. Variations in nip pressure and width can affect sheet moisture content, thickness, and other sheet properties. Excessive pressure and/or width can cause crushing of fibers as well as holes in the resulting paper product. Additionally, the magnitude and area of pressure applied to the sheet may be uneven along or across the length of the roll, often resulting in paper of poor quality having uneven surface characteristics.
Other conditions on a nipped roll can also be important. For example, the stress and strain experienced by the roll cover in the cross machine direction can provide information about the durability and dimensional stability of the cover. In addition, the temperature profile of the roll can assist in identifying potential problem areas of the cover.
It is known to include pressure and/or temperature sensors in the cover of an industrial roll to measure operational parameters while the roll is in use. For example, U.S. Pat. No. 5,562,027 to Moore describes a roll having a plurality of sensors thereon for measuring pressure or temperature at several locations along the roll length and transmitting these measurements to a computer. Also, U.S. Pat. No. 5,699,729 to Moschel et al. describes a roll with a helically-disposed fiber that includes a plurality of pressure sensors embedded in the polymeric cover of the roll.
A conventional system for measuring operating parameters in a nip roll is illustrated in FIG. 1. Referring to FIG. 1, the nip roll 100 includes a cylindrical shell or core 105 and a cover 110 (typically formed of one or more polymeric materials) that encircles the shell 105. A sensing system for sensing pressure, temperature, or some other operational parameter of interest includes a plurality of sensors 120, each of which is embedded in the cover 110. The sensing system also includes a processor 130 that processes signals produced by the sensors 120. A plurality of wires 140 connect the plurality of sensors 120 to a multiplexer 150 and a transmitter 160 mounted on the roll head 170 for transmission to the processor 130.
Typically, two wires per sensor may be required. If multiple sensors are used on a roll, this may require multiple wires running along the length of the roll, which may need to be routed off the face of the roll to electronics units on the head of the roll. These wires may be routed within the layers of the roll cover. As the number of sensors increase, these electrical connections may become increasingly difficult to manage. For example, wire routing can be especially difficult in the case of variable crown rolls. As the head of a variable crown roll may frequently be removed, additional care may be required so that wires are not destroyed. In addition, routing multiple wires through the layers of the roll cover may weaken the integrity of the cover.
As such, several methods of minimizing the number of wires are known. For example, if a common ground wire is used, one wire per sensor (in addition to the ground wire) may be sufficient. Examples of such systems are discussed in U.S. Pat. No. 5,379,652 to Allonen and U.S. Pat. No. 5,383,371 to Laitinen. Further, if only one pressure sensor is in the nip at a time, two wires can be used to support multiple sensors. This is possible because only one sensor may be taking a pressure measurement (and thus transmitting over the wire) at one time. An example of such a system is discussed in U.S. Pat. No. 5,562,027 to Moore.
However, if more than one sensor is in the nip at a time, it may become difficult, if not impossible, to tell which signal is coming from which sensor. For example, for temperature measurements there is a negligible cyclical pulse, so it is nearly impossible to distinguish different temperature sensors using only two wires. Further, for smaller diameter rolls, the sensor spacing may become so close that only a few sensors may be used. In all cases, wire routing may be a difficult task. Therefore, it may be desirable to provide a sensing system that can provide measurements from multiple sensors on the roll using a minimal number of wires.